Isolators in engine accessory belt drives provide a vibration isolation function by utilizing a resilient member between the pulley and hub that is attached to the rotor of the alternator. Since the pulley and hub are connected, the relative motion between these two members is restricted. The stiffness of the resilient member is chose such that the first mode of vibration of the belt drive system is less that the firing frequency of the engine during idling. Therefore, at idle the isolator attenuates the vibration of the pulley, reducing the influence of the pulley on the rotor. Because the rotor vibration is reduced, less torque is required to be transmitted by the pulley and therefore the peak belt tensions are reduced. As a result, there is less chance the tensioner spans will become tight, move the tensioner arm, and slacken the belt span ahead of the alternator in the belt movement direction. This reduces the possibility of belt chirp noises. Isolators are very effective during normal engine operation, but have limited functionality during start-up and shut-down. This is because the system passes through a resonance during start-up and shut down.
To address this problem, decouplers provide a one way clutching feature. During crankshaft pulley acceleration in the engine start-up and running phases, the pulley and hub are locked to each other and the device behaves as a solid pulley. However, during the deceleration phases, the hub can rotate past or “overrun” the pulley. This is useful because it prevents the rotor inertia from creating high tensions in the tensioner span, thereby, avoiding belt slip noise. The decoupler may require a small torque to develop before the device actually overruns. Since there is no connection between the pulley and hub in the overrun mode, the pulley can rotate unrestricted. Decouplers function well for engine start-up and shut down events but are only somewhat adequate during engine running.
Representative of the art is U.S. Pat. No. 5,139,463 which discloses a serpentine belt drive system for an automotive vehicle in which the sequence of driven assemblies includes an alternator assembly comprising a housing and an armature assembly mounted in the housing for rotation about an armature axis. A hub structure is carried by the armature assembly outwardly of the housing for rotation therewith about the armature axis. A coil spring is disposed in operative relation between the alternator pulley and the hub structure for transmitting the driven rotational movements of the alternator pulley by the serpentine belt to the hub structure such that the armature assembly is rotated in the same direction as the alternator pulley while being capable of instantaneous relative resilient rotational movements in opposite directions with respect to the alternator pulley during the driven rotational movement thereof.
What is needed is an isolator having a frictional damping mechanism to damp oscillations between the hub and pulley. The present invention meets this need.